Recently, a magnetic random access memory (hereinafter referred to as an MRAM) that uses the magnetoresistive effect of a ferromagnetic body has been drawing more attention as a next-generation solid-state nonvolatile memory which has a high capacity and which is capable of high-speed reading/writing and capable of operating with low power consumption. In particular, a magnetoresistive element having a ferromagnetic tunnel junction has been drawing attention since the discovery of a high magnetoresistance change rate shown by the magnetoresistive element. The ferromagnetic tunnel junction has a three-layer stack structure comprising a storage layer variable in magnetization direction, an insulator layer, and a fixed layer which faces the storage layer and which maintains a predetermined magnetization direction.
This magnetoresistive element having the ferromagnetic tunnel junction is also referred to as a magnetic tunnel junction (MTJ) element. As a writing method for this element, a writing (spin transfer torque writing) method that uses spin-momentum-transfer (SMT) has been proposed. According to this method, a spin-polarized current is passed through the magnetoresistive element to switch the magnetization direction of the storage layer. The amount of spin-polarized electrons to be injected may be smaller if the volume of a magnetic layer that constitutes the storage layer is smaller. Thus, this writing method is expected to enable both element miniaturization and current reduction.
It has been considered to use, as a ferromagnetic material that constitutes the magnetoresistive element, what is known as a perpendicular magnetization film having a magnetization easy axis (an axis of easy magnetization) in a direction perpendicular to a film plane. When magnetocrystalline anisotropy is used in a perpendicular magnetization configuration, shape anisotropy is not used, so that the element shape can be smaller than that of an in-plane magnetization configuration. Dispersion in a magnetization easy direction can also be reduced. Therefore, the use of a material having high magnetocrystalline anisotropy is expected to enable the maintenance of thermal disturbance resistance and also enable both miniaturization and current reduction.
The problem that arises when the MTJ is formed by the perpendicular magnetization type is that materials included in an underlying layer for adjusting crystal orientation and the storage layer diffuse due to a heat treatment and the magnetoresistance ratio (MR ratio) deteriorates. A technique that addresses this problem has been known. According to this technique, a crystallization accelerating film for accelerating crystallization is formed in contact with an interface magnetic film having an amorphous structure. Thereby, crystallization is accelerated from the side of a tunnel barrier layer, and the interface between the tunnel barrier layer and the interface magnetic film is aligned to achieve a high MR ratio. The use of this technique enables a high MR ratio. However, when a heat treatment associated with element fabrication is taken into consideration, it is preferable that the MR ratio (or resistance value) does not change when a heat treatment is additionally conducted after the initial condition.